1. Field of the Invention
The present invention relates to a GaN-based semiconductor light emitting device, and more particularly, to a GaN-based semiconductor light emitting device, in which decline in internal quantum efficiency resulting from increase in supplied current is inhibited and strain occurring in an active layer due to lattice mismatch is reduced to thereby enhance emission efficiency.
2. Description of the Related Art
In a light emitting device, a material contained therein emits light. As an example, the light emitting device includes a light emitting diode (LED) which has semiconductors together, and converts and outputs an energy resulting from recombination of electrons and holes into light. This light emitting device is broadly used as a lighting device, display and light source and under accelerated development.
Particularly, a mobile phone key pad, side viewer and camera flash using a GaN-based LED which are under active development and utilization, have been commercially viable, thus boosting development of a general lighting apparatus using the LED. A backlight unit of a large-scale TV, a car head light and a general lighting apparatus and other applications evolved from a small portable product to a larger-scale, higher-output, higher-efficiency and more reliable product, thereby requiring a light source demonstrating characteristics necessary for the product.
In manufacturing a gallium nitride (GaN)-based light emitting device, a multi-layer nitride semiconductor thin film having at least two types of nitride thin films deposited therein may be deposited to ensure a desired emission spectrum and emission efficiency.
A material used for an active layer of a GaN-based semiconductor light emitting device adopts an InGan-based tertiary thin film structure. Light emitted from a band gap of an InGaN thin film purportedly has a wavelength covering visible rays, near ultraviolet rays or near infrared rays. To lengthen an emission wavelength in the InGaN film, an In compositional ratio can be increased in the film. However, with a higher compositional ratio of the InGaN thin film, the thin film is significantly degraded in characteristics and reduced in thickness enabling crystallinity to be maintained.
This phenomenon is known to result from phase separation of the tertiary InGaN material. With a higher compositional ratio of In, various characteristics are present in the InGaN thin film. Before phase separation caused by increase in In is completed, the two-dimensional thin film has some portions with different In compositional ratios, that is, suffers In localization. The phase separation can be aggravated by ambient temperature of the thin film or strain induced by lattice mismatch occurring in the multilayer thin film.
Moreover, to lengthen an emission wavelength in the InGaN thin film, the InGaN film may be increased in thickness in place of In compositional ratio. However, a greater thickness of the InGaN thin film deteriorates characteristics of the InGaN thin film. Accordingly, the InGaN thin film is typically formed with a small thickness, thereby experiencing decrease in density of state (DOS) in a quantum well (QW). As a result, a subsequent longer wavelength of the emission spectrum leads to rapid decline in internal quantum efficiency (IQE) caused by increase in supplied current.
Conventionally, the GaN-based semiconductor light emitting device is configured as a double hetero (DH) structure. That is, a layer is formed between one of n-type and p-type semiconductor layers and an InGaN well layer to have an energy band smaller than an energy band of the semiconductor layer and greater than an energy band of an InGaN layer, thereby serving as a charge supply layer to increase DOS. This technology may allow for increase in an amount of charges induced into a well layer but causes stress to be accumulated in the thin film to degrade crystallinity of the thin films sequentially deposited. Notably, in a GaN-based semiconductor light emitting device including a plurality of well layers, stress is continuously aggravated to undermine emission efficiency despite increase in DOS.